This application claims priority to United Kingdom patent application GB 9709967.5, filed May 17, 1997, which was filed as International patent application PCT/GB98/01418 on May 15, 1998, and which was published as International publication WO 98/52495A1 on Nov. 26, 1998.
This invention concerns prosthetic grafts for use in vascular surgery, particularly for by-passes to relatively small arteries.
By-passes required to save limbs can be long, say going from groins to below knees, and to arteries that may be as small as 1 to 5 mm in diameter. Where patients have no other veins that can be used, as is often the case with patients having relevant serious conditions, the only positive alternative is to use prosthetic grafts of synthetic materials, for example flexible tubes of polytetrafluoroethylene (PIFE). Simple direct end connections or anastomosis of prosthetic graft tubes (usually run at an acute angle or more or less parallel with the artery and end cut at an angle) to side apertures in arteries, perhaps particularly arteries substantially less than 5 mm in diameter, has unfortunately been followed by formation of fibrous intimal hyperplasia, which leads to serious blood flow reduction and even stoppage. The fibrous intimal hyperplasia occurs in regions within and around the graft connection, where there is little or no shear stress between the blood flow and the graft and arterial walls.
It is known to use a small piece of natural vein to make a short cuff known as the Miller cuff, that is joined by surgical stitching to and between the artery opening and the end of the prosthetic graft tube. Improved success rates for indirect prosthesis-to-vein-to-artery connection, compared with direct prosthesis-to-artery, have involved reduced adverse effect from intimal hyperplasia. Contributory factors, for cuff type and other prosthesis types, have been considered and postulated as including reducing tendencies to turbulence of blood flow, and/or optimising approximation to laminar blood flow, and/or for suppleness of the natural vein parts to aid absorption or cushioning blood pulsing. These factors have further been seen particularly as contributing to avoiding or minimising occurrence of artery wall shear stress. However, fibrous intimal hyperplasia still occurs with the so called Miller cuff because regions of flow separation and low shear stress still occur within the cuff.
U.S. Pat. No. 5,156,619 discloses a vascular prosthesis comprising a tube of material other than autologous vascular tissue, the tube having an enlarged end formation for surgical connection direct to an opening formed in an artery, the formation having a heel and a toe at opposite ends of a first longer diameter parallel to the axis of the tube and a second shorter transverse diameter.
WO-A-9731591 discloses a flanged graft for end-to-side anastomosis grafting having an integral terminal flanged skirt or cuff, which facilitates an end-to-side anastomosis directly between an artery and the expanded flange bypass graft without need for an intervening venous collar or venous patch.
It has been proposed to provide a vascular prosthesis comprising a tube of synthetic material having an end formation for surgical connection directly to an opening formed in an artery, the end formation comprising an enlarged chamber serving to promote localised movement of blood having a non-laminar nature with a shear stress inducing relationship to the arterial wall. The enlarged chamber has a convex outer wall. Further experimentation has revealed that this type of vascular prosthesis, whilst representing an improvement on the Miller cuff is still not ideal.
An object of this invention is to provide an improved vascular prosthesis for use in vascular surgery.
According to the present invention there is provided a vascular prosthesis comprising a tube of material other than autologous vascular tissue, said tube having an end formation for surgical connection direct to an opening formed in an artery, said formation comprising an enlarged chamber having a heel and a toe at opposite ends of a first longer diameter parallel to the axis of the tube and a second shorter transverse diameter, characterised in that transition between the tube and the toe is outwardly initially convex before a final concave portion (64), whereby said enlarged chamber serves to promote localised movement of blood having a non-laminar nature with shear stress inducing relationship to receiving arterial wall.
The heel of the enlarged chamber is formed at one end of the longer diameter, and the transition between the tube and the heel is preferably generally concave.
Transition between the tube and opposite ends of the shorter diameter is preferably outwardly convex.
It is also preferable that the tube have a narrower portion prior to transition to the enlarged chamber. It is believed that such narrowing of the tube will increase blood velocity entering the enlarged chamber of the prosthesis and hence increase shear stress in that region. The vascular prosthesis of the invention is intended to promote vertical blood flow in the region of its arterial connection in order to reduce or eliminate regions of low shear stress and regions of long residence times where blood elements can accumulate in the region of the graft connection.
The grafts of the invention are preferably made of plastics material, especially polytetrafluoroethylene (PTFE).
The term xe2x80x9cnon-laminarxe2x80x9d as used herein is intended to define blood flow other than parallel to arterial walls and, in particular, includes localised laminar movement of blood having significant secondary components.
Separation of flowing blood from the inner wall of the tube near its enlarged chamber, and associated with non-laminar flow, is preferably such as to produce a swirling action that may include locally circulatory or recirculatory movement of blood, further preferably in the nature of or including a vortex action. Such blood flow separation will usually be at and adjacent preferred acute angling of the prosthesis tube for its direct connection to the artery, say at least partially within the enlarged chamber.
A preferred end chamber of the prosthesis tube of the invention is an enlargement which produces blood flow characteristics therein that result in an increase in wall shear stress.
Desired non-laminar blood flow promotion is preferably effective only in phases of cycles of blood-flow pulsing, which phases preferably alternate with other phases of more laminar flow sufficient to assist flow of all blood into the artery away and from that end of the prosthesis. The pulsed nature of normal blood flow involved successive time-spaced rises in pressure. Each pressure rise preferably causes both an initial relatively smooth or laminar blood flow in and out of the prosthesis-to-artery connection and a later transition into desired non-laminar blood movement. The preferred non-laminar vortex type movement preferably collapses before the next pressure rise.